It has previously been proposed to remove sulfur dioxide from a combustion exhaust gas with conversion to gypsum by scribbing with an alkali sulfite aqueous solution to form an alkali bisulfite-containing aqueous solution and then reacting with a calcium compound. The reaction of sulfur dioxide with the aqueous sulfite solution, produces an alkali bisulfite-containing aqueous solution, to which is then added a calcium compound such as slaked lime (calcium hydroxide) or calcium carbonate to form calcium sulfite. This calcium sulfite is, after separation, then oxidized into gypsum (calcium sulfate). The filtrate from the calcium sulfite separation process is an alkali sulfite aqueous solution and is recirculated to the sulfur dioxide absorption vessel. To oxidize the calcium sulfite, water is added to the calcium sulfite to form a slurry into which oxygen is injected to form gypsum. The filtrate removed from the gypsum separation process is water containing alkali sulfate in some quantity which, together with the filtrate removed from the calcium sulfite separation operation, is used as an alkali sulfite aqueous solution for absorbing sulfur dioxide. Calcium sulfite maay be slurried by mixing with the filtrate from the gypsum filter, thus avoiding the need to supply water from an external source. This method will permit an adjustment of the water balance within the system. Another feature of the foregoing process is that it is effected within a closed system.
Where sodium sulfite is used as the alkali sulfite and calcium hydroxide as the calcium compound, the reaction mechanism of the foregong method may be expressed by the following reaction formulas (1), (2) and (3): EQU Na.sub.2 SO.sub.3 + SO.sub.2 + H.sub.2 O .fwdarw. 2NaHSO.sub.3 TM (1) EQU 2naHSO.sub.3 + Ca(OH).sub.2 .fwdarw. CaSO.sub.3 + Na.sub.2 SO.sub.3 + 2H.sub.2 O (2) EQU caSO.sub.3 + 1/2 O.sub.2 .fwdarw. CaSO.sub.4 TM (3)
a disadvantage of the foregoing method is that, during the reaction between the alkali sulfite aqueous solution and sulfur dioxide contained in the combustion exhaust gase, a portion of the alkali sulfite will be oxidized with oxygen contained in the combustion exhaust gas to form an alkali sulfate which will be accumulated in the alkali bisulfite-containing aqueous solution. The increase in the quantity of the accumulated alkali sulfate represents a reduction in the quantity of alkali sulfite available for reaction with the sulfur dioxide. This in turn will reduce the efficiency of removal of the sulfur dioxide from the combustion exhaust gas. To eliminate this disadvantage it has been proposed to remove the accumulated alkali sulfate from the alkali bisulfite-containing aqueous solution, for example, by externally adding sulfuric acid and calcium sulfite to said aqueous solution. Thus, the accumulated alkali sulfate will be converted into gypsum which may be separated. This reaction is expressed by the following formulas (4) and (5), wherein sodium sulfate employed as the alkali sulfate. EQU H.sub.2 SO.sub.4 + CaSO.sub.3 .fwdarw. CaSO.sub.4 + SO.sub.2 + H.sub.2 O (4) EQU so.sub.2 + na.sub.2 SO.sub.4 + CaSO.sub.3 + H.sub.2 O .fwdarw. CaSO.sub.4 + 2NaHSO.sub.3 (5)
the filtrate removed from the gypsum separation process is an alkali bisulfite-containing aqueous solution, which can be reused within the system and thus the entire process can be effected within a closed system.
In the foregoing reactions carried out within a closed system, it is inevitable, however, that various metal ions are introduced into the system. These metals may include a slight amount of nickel originating in the heavy fuel oil and introduced into the system by the combustion exhaust gas and other metals such as magnesium and iron introduced with the calcium compound. The presence of such metallic ion impurities reduces the efficiency of sulfur dioxide removal. This is because the metallic ion impurities will be converted into sulfates or sulfites, and then oxidized into sulfates by the oxygen contained in the combustion exhaust gas and also by a portion of the oxygen introduced for the purpose of oxidizing calcium sulfite to gypsum, and the metallic sulfates will, in turn, react with the alkali sulfite contained in solution, forming alkali sulfates. It is believed that the foregoing series of reactions proceed in the manner indicated by the following reaction formulas (6), (7) and (8), wherein the metallic ion impurity is expressed as Mg.sup.+.sup.+ by way of example and sodium sulfite is the alkali sulfite. EQU Mg.sup.+.sup.+ .fwdarw. MgSO.sub.4 or MgSO.sub.3 ( 6) EQU mgSO.sub.4 + Na.sub.2 SO.sub.3 .fwdarw. Na.sub.2 SO.sub.4 + MgSO.sub.3 ( 7) EQU mgSO.sub.3 + 1/2O.sub.2 .fwdarw. MgSO.sub.4 ( 8)
as metallic ion impurities in increasingly larger quantities are accumulated in the system, accumulated alkali sulfate will also increase in quantity and accordingly the alkali sulfite available for reaction with sulfur dioxide will be correspondingly reduced in quantity. Experiments indicate that the double decomposition reaction, which is solid-liquid heterogeneous reaction between, for example, NaHSO.sub.3 and Ca(OH).sub.2 as expressed by formula (2), is greatly impeded by the presence of several hundreds ppm of Ni.sup.+.sup.+ in the alkali bisulfite-containing aqueous solution. To avoid the danger that the metallic ion impurities will accumulated in the system, it may be necessary to discard the filtrate the gypsum separation step. Discarding the filtrate, however, makes it impossible to carry out the sulfur dioxide removal within a closed system and the useful alkali valves, once reacted with sulfur dioxide, are wasted without being reused. Also, the disposed filtrate will pose a potential environmental pollution problem.
In view of the foregoing, in the removal of sulfur dioxide from a combustion exhaust gas and conversion to gypsum by treating, within a closed system, the sulfur dioxide containing combustion exhaust gas with an alkali sulfite aqueous solution and a calcium compound, it is preferred that metallic impurities be removed from the system.